Monitoring apparatus and method particularly useful in photolithographically processing substrates

ABSTRACT

Apparatus for processing substrates according to a predetermined photolithography process includes a loading station in which the substrates are loaded, a coating station in which the substrates are coated with a photoresist material, an exposing station in which the photoresist coating is exposed to light through a mask having a predetermined pattern to produce a latent image of the mask on the photoresist coating, a developing station in which the latent image is developed, an unloading station in which the substrates are unloaded and a monitoring station for monitoring the substrates with respect to predetermined parameters of said photolithography process before reaching the unloading station.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of parent application Ser. No.11/924,365, filed Oct. 25, 2007 (now U.S. Pat. No. 7,525,634), which isa continuation of parent application Ser. No. 11/402,009, filed Apr. 12,2006 (now U.S. Pat. No. 7,289,190), which is a continuation of U.S.application Ser. No. 10/763,383, filed Jan. 26, 2004 (now U.S. Pat. No.7,030,957), which is a continuation of U.S. application Ser. No.09/730,919, filed Dec. 6, 2000 now U.S. Pat. No. 6,842,220, which is acontinuation of U.S. application. No. 09/184,727 (now U.S. Pat. No.6,166,801), filed Nov. 2, 1998, and claiming priority from Israelapplication No. 125337, filed Jul. 14, 1998. The entire contents ofwhich is hereby incorporated by reference.

FIELD OF THE INVENTION

The present invention relates to inspection apparatus and methodsparticularly useful in photolithographically processing substrates. Theinvention is especially useful in producing semiconductors, and istherefore described below with respect to this application.

BACKGROUND OF THE INVENTION

The principal process of semiconductor production is photolithography,which includes three main serial steps or operations:

-   -   (a) coating the wafer with photoresist material (PR);    -   (b) exposing the PR through a mask with a predetermined pattern        in order to produce a latent image of the mask on the PR; and    -   (c) developing the exposed PR in order to produce the image of        the mask on the wafer.

The satisfactory performance of these steps requires a number ofmeasurement and inspection steps in order to closely monitor theprocess.

Generally speaking, prior to a photolithography process, the wafer isprepared for the deposition of one or more layers. After aphotolithography process is completed, the uppermost layer on the waferis etched. Then, a new layer is deposited in order to begin theaforementioned sequence once again. In this repetitive way, amulti-layer semiconductor wafer is produced.

FIG. 1 schematically illustrates a typical set-up of photocluster toolsof the photolithography process in a semiconductor fabrication plant(Fab). The photocluster (or link) is composed of two main parts: thephototrack 5, and the exposure tool 8. The phototrack includes a coatertrack 6 having a cassette load station 6 a, and a developer track 10having a cassette unload station 10 a. Alternatively, both coater anddeveloper functions may be combined and realized in the same stations(not shown). The wafer W is placed in the cassette station 6 a. Fromthere the wafer is loaded by a robot 2 to the coater track 6 where thecoating step (a) commences. After step (a), the wafer is transferred bythe robot 2 to the exposure tool 8, where the exposing step (b) isexecuted. Here, using optical means installed inside the exposure tool,the pattern on the mask is aligned with the structure already on thewafer (registration). Then, the wafer W is exposed to electromagneticradiation through the mask. After exposure, robot 2 transfers the waferto the developer track 10 where the micro-dimensional relief image onthe wafer is developed (step). The wafer W is then transferred by robot2 to the cassette station 10 a. Steps (a)-(c) also involve severaldifferent baking and other auxiliary steps which are not describedherein.

As shown in FIG. 1, the coater track 6, the exposure tool 8, and thedeveloper track 10, are tightly joined together in order to minimizeprocess variability and any potential risk of contamination duringphotolithography, which is a super-sensitive process. Some availablecommercial exposure tools are series (MA-1000, 200, 5500) of DainipponScreen MFG. Co. Ltd., Kyoto, Japan, PAS-5500 series of ASM Lithography,Tempe, Ariz., series FPA 3000 and 4000 of Canon USA Inc., USA, andMicroscan of SVGL, Wilton, Conn. Some available phototracks are series90s and 200 of SVGT, San-Jose, Calif., Polaris of FSI International,Chaska, Minn., and phototracks D-spin series (60A/80A, 60B, 200)) ofDainippon Screen MFG. Co. Ltd., Kyoto, Japan, Falcon of FairchildTechnologies Inc., USA and of Tokyo Electric Laboratories (TEL), Japan.

It is apparent that in such a complex and delicate production process,various problems, failures or defects, may arise or develop during eachstep, or from the serial combination of steps. Because of the stringentquality requirements, any problem which is not discovered in time mayresult in the rejection of a single wafer, or of the whole lot.

A wafer cannot be taken out of the photocluster for measurement orinspecting before the process is completed and the wafer arrives at thecassette station 10 b. As a result, any process control based onmeasuring processed wafers cannot provide ‘real time’ processmalfunction detection. Therefore, there is an urgent need for anapproach based on integrated monitoring, i.e., a monitoring apparatusphysically installed inside or attached to the relevant production unit,dedicated to it, and using its wafer handling system. Such integratedmonitoring can provide tight, fast-response and accurate monitoring ofeach of the steps, as well as complete and integrated process controlfor the overall semiconductor production process, in general, and forphotolithography, in particular.

However, examination of the prior art, insofar as known to us, indicatesthat no such monitoring and control methods and/or systems exist.Rather, only ‘stand-alone’ monitoring systems appear to be available atthe moment.

A ‘Stand-alone’ monitoring system is one which is installed outside theproduction line and in which the wafers are transferred from theproduction unit to the monitoring system using a separate handlingsystem than that of the production process.

In general, three different monitoring and control processes areperformed at the present time during semiconductor fabrication process.These are monitoring of (a) overlay misregistration, (b) inspecting and(c) critical dimension (CD) measurement. A brief description of each ofthese processes is given below:

(a) Overlay Registration Control

The overlay registration (hereinafter—“overlay”) is a process executedin the exposure tool 8 in which the pattern on the mask is aligned withrespect to the pattern features existing already on the uppermost layeron the wafer. The shrinking dimensions of the wafer's features increasesthe demands on overlay accuracy.

An overlay error or misregistration (hereinafter—“overlay error”) isdefined as the relative misalignment of the features produced by twodifferent mask levels. The error is determined by a separate metrologytool from the exposure tool.

FIG. 2( a) illustrates a typical overlay error determination site on awafer. It is composed of two groups of target lines, one on theuppermost feature layer of the wafer 11 and the second is produced onthe new PR layer 16. Target lines 16 are similar but smaller than targetlines 11; thus they can be placed in the center of target lines 11.Therefore, these overlay targets are called “bars in bars”. FIG. 2( b)is a top view of the same overlay error determination site. The lines ofthese targets, such as 11 a and 16 a are typically of ˜2 μm width, and10-15 μm length, respectively.

According to a common method, the overlay error is defined as therelative displacement of the centers of target lines 11 with respect tolines 16, in both the x and y axis. For example, in FIG. 6 thedisplacements between lines 11 a and 16 a, 11 b and 16 b are denoted as14 a and 14 b, respectively. Thus, the overlay error in the x axis isthe difference between the lengths of lines 14 a and 14 b.

FIG. 3 illustrates a common configuration of photocluster tools and a‘stand-alone’ overlay metrology system composed of a measurement unitand an analysis station. It should be noted that wafers to be examinedare taken out of the photolithography process-line, and handled in themeasurement tool. The latter results from the facts that with theavailable overlay technology (i): closed loop control in ‘real time’ isnot possible; (ii) not all the wafers as well as all the layers within awafer are measured for overlay error; (iii) additional process step isneeded; and (iv) a ‘stand alone’ tool is needed. It should be noted thatit is a common situation in the Fab, especially in advanced productionprocesses, that during ‘stand alone’ overlay measurement, the processingof the lot is stopped. This break may even take a few hours.

The results of the measurements are sent to the analysis station, and afeedback is returned to the stepper in the photocluster tool.

U.S. Pat. No. 5,438,413 discloses a process and a ‘stand-alone’apparatus for measuring overlay errors using an interferometricmicroscope with a large numerical aperture. A series of interferenceimages are tat different vertical planes, and artificial images thereofare processed, the brightness of which is proportional to either thecomplex magnitude or the phase of the mutual coherence. The differencesbetween synthetic images relating to target attribute position are thenused as a means of detecting overlay error. KLA-Tencor, Calif., theassignee of this patent, sells a ‘stand-alone’ machine under the brandname KLA-5200. In this system, the measurement and the analysis stationare combined together.

U.S. Pat. No. 5,109,430 discloses another overlay metrology system. Bycomparing spatially filtered images of patterns taken from the waferwith stored images of the same patterns, the overlay error isdetermined. Schlumberger ATE, Concord, Mass., the assignee of thispatent, supplies a ‘stand-alone’ machine for submicron overlay under thebrand name IVS-120.

Other ‘stand-alone’ overlay metrology systems are manufactured byBIO-RAD micromeasurements, York, Great Britain, under the brand nameQuestar Q7, as well as by Nanometrics, Sunnyvale, Calif. (Metra series).

All the aforementioned methods and metrology systems for determiningoverlay error suffer from several drawbacks including the following:

1) They are all ‘stand-alone’ systems, i.e., operating off-line thephotolithography process. Thus, they provide post-process indication ofoverlay errors, not during the production process itself, or before abatch of wafer production is completed. In some cases this may takehours, or more.

2) They result in a waste of wafers, and/or lots of wafers because ofthis post-process response. This results from the continuous operationof the photolithographic process on the one hand, and the time-delayfrom the time a wafer is sent offline to overlay measurements until aresponse about an error is obtained, on the other hand.

3) Usually, one of the main overlay error sources is the first maskalignment for a wafer to come on a lot. Such an error source cannot becorrected later since the error varies for each lot. For this reason itis important to have the feedback within the time frame of the firstwafer which cannot be obtained using a ‘stand-alone’ tool.

4) The overlay sampling frequency is limited due to contaminationrestrictions and additional expensive time needed for extra handling andmeasurements.

5) Throughput of the photolithography process is reduced as a result ofthe post-process overlay detection and the long response-time, as wellas of the reduced sampling frequency mentioned in (3).

6) These stand-alone systems require additional expensive foot-print andlabor in the Fab.

7) The microlithography tools are the “bottle neck” in thesemiconductors production process and they are the most expensive toolsin the FAB. Their partial utilization due to late off-line measurementsreduces drastically overall equipment efficiency in the Fab.

(b) Inspecting

Inspecting during the production of semiconductors wafers can be definedas a search for defects caused by:

-   -   (a) contamination (dirt, particles, chemicals, residues, etc.),        and/or    -   (b) process induced failures related to PR, coating, baking,        development, handling, etc.

In order to detect defects originating only from the lithographyprocess, a specific inspecting step is conducted after the developmentstep as illustrated in FIG. 4. It is usually called “after developmentinspecting” (ADI), or “post-development check” (PDCK). The presentinvention is mainly relevant for ADI.

In general, data obtained during the inspecting is analyzed, and in casean increased defects level is detected, an alarm is sent to theengineering level or to the production line. Once again it should benoted that, as in the case of overlay metrology, with the currenttechnology, the ADI is located out of the production line; i.e., wafersto be inspected are taken out of the production process and handled in aseparate inspecting station. It should also be noted that it is a commonsituation in the Fab, especially in advanced production processes, thatduring ‘stand alone’ inspecting, the processing of the lot is stopped.This break may take even few hours.

Today, the majority of ADI activities are non-automatic visualinspecting conducted by humans. In particular, no integrated automaticADI system is commercially available at the moment.

ADI is aimed at:

(i) Coarse inspecting—A wafer is handled by hands and is visuallyinspected by eye-sight for large defects. These defects can be, forexample, poor spinning during coating, poor development, scum,non-attached PR (‘lifting’), and/or edge beads. This method can usuallyonly detect defects bigger than tens of microns.

(ii) Fine inspecting—predetermined sites or targets on a wafer arevisually inspected with the aid of a microscope (20-50× magnification).

These defects can be, e.g., shorts between conducting lines, and focusfailures of the stepper.

ADI conducted by humans has several disadvantages:

(a) It is tedious and requires great concentration to locate patterndiscrepancies in repetitive and complex circuits.

(b) The results are not uniform with respect to each inspector as wellas between different inspectors. This point becomes crucial whenconsidering the increased importance of inspecting at times when thewafer features become more and more delicate due to the continuousshrinking of the wafer's features.

(c) It is not a consistent means for statistical analysis and formeasuring process quality due to non-repetitive results.

(d) Additional costs due to the labor.

(e) Non objective inspecting, neither in defects identification nor inthe specific action which should take place once a specific defect isidentified.

(f) Fluctuating throughput results, among other things, in difficultiesto determine sampling frequency.

(g) Manual inspecting is also done off-line and therefore suffers fromall the same aforementioned disadvantages of ‘stand-alone’ systems.

To complete the picture, it should be noted that two automatic opticalinspection (AOI) methods for defect detection are known, but their highcost and low throughput limit their use in actual production.

(i) Absolute methods—illuminating a wafer at a predetermined angle(“grazing”) and collecting the reflected signal from the wafer's plane.Any signal above a threshold (absolute) value is determined as a defect.According to this method, particles bigger than 0.1 μm can be detected.

(ii) Comparative methods—these are divided into ‘die to die’ and ‘die todatabase’. A wafer is photographed and then an automatic comparison ofpixels in one die is made with respect to the correlative pixels in aneighbor die, or to a database. Usually, the result of the comparisonshould fit a set threshold, unless there is a defect. The threshold maybe a function of the gray level and/or the specific location of the dyein the wafer.

Method (ii) overcomes the shortcomings of method (i), and usuallydetects defects such as dirt particles (>0.1 μm), bridging of conductinglines, missing features, residues of chemicals and PR, etc. The defectlevel these methods can detect is determined according to the designrule of the industry (e.g., 0.1 μm).

None of the available inspecting tools samples each wafer, but onlyseveral wafer in a lot. Moreover, the lack of such inspecting systemsprevents any option for automatic and tight feedback or closed loopcontrol over the lithography process. Thus, any serious attempt forestablishing or even improving the process control around thephotolithography process is prevented, or at least is met with crucialobstacles due to the lack of such method(s) and systems.

Critical Dimension (CD) Control

A third monitoring and control process is the Critical Dimension CDcontrol which includes measurements of characteristic dimensions incritical locations on a wafer, e.g., widths of representative lines,spaces, and line/space pairs on the wafer. CD metrology tools are basedon two main technologies: the CD scanning electron microscope (CD SEM),and the atomic forced microscope (CDAFM). Commercial tools based on CDSEM are series 7830XX of Applied Materials, Santa Clara, Calif., andDEKTAK SXM-320 of VEECO, USA is based on AFM.

FIG. 5 illustrates common configurations of ‘stand-alone’ CD tools withthe production process. Typically, CD measurements take place after thedeveloping step and/or after etching. The CD tool is located out of theproduction line, i.e., wafers to be measured are taken out of theproduction process and handled to a separate CD station. It should benoted that it is a common situation in the Fab, especially in advancedproduction processes, that during ‘stand alone’ CD measurement, theprocessing of the lot is stopped. This break may take even few hours.

In general, data obtained during the CD measurement is analyzed, andthen a kind of feedback (or alarm in a case of a width out of thepermitted range) is sent to the relevant unit in the production line.

CDSEM and CDAFM allow CD measurement for line/space width below theresolution limit of optical microscope. However, when possible, opticalCD (OCD) measurement may be very useful because they can be combinedwith optical overlay measurement systems. Recently, (C. P. Ausschnitt,M. E., Lagus (1998) Seeing the Forest for the Trees: a New Approach forCD Control”, SPIE, vol. 3332, 212-220), it was proposed to use OCD evenfor sub-micron design rules that is behind the optical resolution. Theidea is that optical systems allow fast measurement of many linessimultaneously. Statistical treatment of multiple measurements with lowaccuracy, allows to extract such important manufacturing data asrepeatability or deviation trends.

It is clear, as was noted before with respect to overlay metrology andinspecting tools, that since all CD metrology systems are ‘stand-alone’tools, they suffer from the same drawbacks as discussed before.Moreover, especially in the case of CD measurement, the results, e.g.,line width, give a limited ability to correlate the measurement to anyspecific cause.

Methods for Process Control in Lithography

Overlay and CD monitoring can be performed in various levels in order toestablish process control. The first common level is “lot to lotcontrol”. In this method each lot is a basis for the next lot to run inthis process. Small correction can be made by considering the results ofthe previous lot and making corrections. However a certain increment inthe risk is introduced because a total lot may be lost.

A second control level is “send ahead wafer”. In this method a pilotwafer is sent through the coating-exposure-developing steps, exposed inthe recommended exposure, and is then sent to CD measurement.Satisfactory results will be a basis for the set up conditions of thelot, whereas unsatisfactory results will cause another wafer to beexposed with corrections for the exposure conditions. The over allsequence of a “send ahead wafer” control can take many hours whilevaluable utility time of the production tools, as well as the productionlot, may be lost.

In some cases there is a need for a higher control level. This may beperformed in a full process window mapping by running an exposurematrix, or focus exposure matrix, and analyzing the results. However,this is the most time-consuming method.

The drawbacks of these methods, when conducted with ‘stand-alone’overlay and OCD tools, are that they are time and effort consuming andthey usually do not respond directly for certain causes, or do notreveal any problematic sources. However, they make the “time to respond”shorter as compared to long-term trend charts. Nonetheless, to enable areal feedback to problems, there is a crucial need for integratedmonitoring of the process steps. The on-line measurements can responddirectly to a certain cause with the correct straight forward correctionaction.

It should be emphasized that these problems with respect to processcontrol ‘stand-alone’ systems are dramatically aggravated whenconsidering the coming future developments in the semiconductorindustry. Because of the shrinking critical dimensions of the wafer'sfeatures, as well as the introduction of new and non-stable processes(e.g., DUV resist, and transition to 300 mm diameter wafer withcorresponding restrictions on wafer handling), the need for anintegrated monitoring and process control for semiconductors productionbecomes crucial. For this reason, traditional process control methodsthat use long-term trend charts, and which are “offline methods” will bemore and more excluded.

As noted before, integrated monitoring and process control systems are areasonable solution for the above discussed problem. However, such asystem should be considered from several aspects and meet specificrequirements in order to become real and feasible:

-   -   (a) Small footprint—such a system should have as small footprint        as possible (practically not smaller than the wafer size) in        order to be physically installed inside the photocluster;    -   (b) Stationary wafer—the wafer should be stationary during        inspection and measurement to exclude extra wafer-handling;    -   (c) High throughput—the system should have high throughput such        as not to reduce the photocluster throughput;    -   (d) Cleanliness—the measuring unit should not interfere in any        way with the photocluster or introduce any potential risk of        contamination;    -   (e) Access for maintenance—the system parts except the measuring        unit (e.g., control electronics, light source), should be        outside the photocluster in order to enable, among other things,        easy and quick maintenance without any disturbance to the        photocluster;    -   (f) Cost-effective—the integrated tool cost should be a small        portion of the phototrack cost;

“Stand-alone” monitoring and process control systems do not meet thesestringent requirements, and apparently cannot be used as an integratedsystem. Moreover, no such integrated system is now available on themarket. Therefore, there is a need for a new monitoring and processcontrol apparatus and method having advantages in the above respects.

OBJECTS AND BRIEF SUMMARY OF THE PRESENT INVENTION

An object of the present invention is to provide a novel apparatus andmethod having advantages in one or more of the above-described respects,particularly important in the photolithography processing of substrates,e.g., semiconductors wafers.

According to one aspect of the present invention, there is providedapparatus for processing substrates according to a predeterminedphotolithography process, comprising: a loading station in which thesubstrates are loaded; a coating station in which the substrates arecoated with a photoresist material; an exposing station in which thephotoresist coating is exposed to light through a mask having apredetermined pattern to produce a latent image of the mask on thephotoresist coating; a developing station in which the latent image isdeveloped; and an unloading station in which the substrates areunloaded; characterized in that said apparatus further comprises amonitoring station for monitoring the substrates with respect topredetermined parameters of said photolithography process before beingunloaded at the unloading station.

As will be described more particularly below, the optical inspectionsystem between the developing station and the unloading station maydetect one or more of the following: (a) overlay registration errors;(b) defects in the photoresist layer; and/or (c) critical dimensionalerrors.

According to further features in the described preferred embodiments,the inspecting station includes: a supporting plate between thedeveloping station and the unloading station for receiving substrates tobe inspected; a sealed enclosure between the developing station and theunloading station and having a transparent window aligned with andfacing the supporting plate; an optical inspecting system within thesealed enclosure for inspecting substrates on the supporting plate viasaid transparent window; and a light source for illuminating thesubstrates via the optical inspecting system.

In the described preferred embodiments, the light source is externallyof the sealed enclosure and produces a light beam which is applied tothe optical system within the sealed enclosure.

In addition, the optical inspection system within the sealed enclosureincludes an optical image device; and the inspecting station furtherincludes a digital image processing unit externally of the sealedenclosure and connected to the optical imaging device by electricalconductors passing into the sealed enclosure.

The inspecting station further includes a central processing unitexternally of the sealed enclosure and connected to the opticalinspecting system for controlling the system via electrical conductorspassing into the sealed enclosure.

According to still further features in the described preferredembodiments, the optical inspecting system within the sealed enclosureincludes: a low-magnification channel for aligning the opticalinspecting system with respect to a patterned substrate on thesupporting plate or for coarse inspection; and a high-magnification orhigh-resolution channel for measuring the predetermined parameters ofthe photolithography process after the substrate has passed through thedeveloping station and before reaching the unloading station. Thelow-magnification channel and the high-resolution channel are fixed withrespect to each other.

The invention also provides a novel method of processing substratesaccording to a predetermined photolithography process having advantagesin the above respects.

As will be described more particularly below, the invention permits oneor more or the following to be provided:

1) An integrated apparatus for overlay metrology and/or inspecting.and/or OCD measurements. Such a system would have high accuracy and highthroughput and could be physically combined inside the presentfoot-print of photocluster tools; i.e., it would have a zero additionalfoot print on the production floor. The combination of up to threedifferent functions in one tool would have its own advantages: (i)Better exploitation of utilization time—each inspecting, overlay and CDmeasurement could have its own sampling frequency and need not be thesame for every wafer. Thus, such a system could be continuously operatedwhile its utilization time is shared between the three functions. (ii) Adirect result from (i) is that the system could drastically decrease thenumbers of lots which would be simultaneously running around theproduction process as common today (one for overlay measurement, one forinspecting, one for CD, and one which begins the lithography process),(iii) Such apparatus could be client-oriented, i.e., could be exactlyfitted to the customer needs as well as to changing needs. (iv) Thesystem could be oriented for a specific problem; (v) The system couldhave a relatively low price;

2) An apparatus as in (1) which will not reduce the throughput of thephotocluster by carrying out measurements/inspection in parallel withthe processing of the next wafer.

3) A modular apparatus as in (1) capable to perform only overlaymetrology or inspecting or OCD measurements with enhanced performance ifneeded.

4) An apparatus as in (1) combined with a processing unit, thus enablingto establish monitoring and process control based on overlay, inspectingand OCD measurements.

5) An integrated automatic inspecting system which is much moreaccurate, faster, and repetitive than visual inspecting conducted byhumans.

6) A method for integrated monitoring and process control for overlaymetrology and/or inspecting and/or OCD within the photolithographyproduction process, thereby enabling much shorter response times ascompared to the common ‘stand-alone’ systems.

7) Methods which facilitate and dramatically shorten the time needed forcommon process control methods such as ‘send a wafer ahead’, ‘lot tolot’, etc.

8) A new and practical monitoring and process control method by means of‘wafer to wafer’. Such a method is practically impossible to conduct inthe current situation of ‘stand-alone’ systems. However, with these newintegrated methods, which do not decrease the throughput of theproduction process a tight, fast-responding, directly to cause,monitoring and process control method can be conducted. Apparently, in acertain circumstances these new method may save the need for separateand expensive ‘stand-alone’ systems for overlay metrology, inspectingand OCD.

9) A method with which both overlay error, inspecting and OCD can bedetermined either on one wafer of a lot or on different wafers in thesame lot.

10) An integrated and elaborated inspecting monitoring and controlsystem.

1. The process can be implemented in several alternative levels. Onewould be aimed at notifying the production process controller aboutdefects in general. Another would be aimed at investigating the directreasons and/or the induced failure which caused the defects. Then, afeedback would be directed, to any predetermined relevant point in theproduction process. By this, a feedback or a closed loop control can beestablished either for a single step in the production process (e.g.,exposure step, post exposure bake (PEB)) or for the whole process whencombined with other metrology system(s), such as overlay and/or criticaldimension metrology systems.

Such a method and apparatus have the potential to save expensiveutilization time (e.g., by shortening methods such as ‘send a waferahead’) as well as diminishing the amount of test wafers wasted duringthe production process.

In addition, there would be no need for additional wafer handling fromor to the photolithography tools, thus saving utilization time as wellas preventing additional contamination and wafer breakage.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is herein described, by way of example only, withreference to the accompanying drawings, wherein:

FIG. 1 is typical apparatus set-up of photocluster tools as presentlyused in a photolithography process of a semiconductor fabrication plant;

FIGS. 2A and 2B are schematic illustrations of an overlay target;

FIG. 3 is a block diagram illustrating a common configuration ofphotocluster tools and a ‘stand-alone’ overlay metrology system;

FIG. 4 is a block diagram illustrating possible points where afterdeveloping, inspecting can be introduced with respect to semiconductormulti-layer production process;

FIG. 5 is a block diagram illustrating common configurations of‘stand-alone’ CD tools presently used in the production process;

FIG. 6 is a schematic illustration of one manner in which thephotolithography cluster tools in the apparatus of FIG. 1 may becombined with an integrated overlay/inspecting/OCD tool in accordancewith the present invention;

FIG. 7 is a schematic side view of an integrated lithography monitoringsystem in the apparatus of FIG. 6;

FIG. 8 is a schematic illustration of a preferred embodiment of thepresent invention used as an integrated overlay metrology tool;

FIG. 9 is a schematic illustration of one form of optical system thatmay be used in this apparatus of the present invention;

FIG. 10 is a schematic illustration of one manner that may be used formeasuring the angle between the incident ray of the high-resolutionchannel to the wafer surface;

FIG. 11 is a view along the section lines A-A in FIG. 2B;

FIG. 12 is a schematic illustration of the gray level of target lines 11a and 16 a (FIG. 11);

FIGS. 13A and 13B are schematic illustrations of the gray levels ofimages of a ‘knife edge’ pattern and their line spread function (LSF)functions;

FIG. 14 is a schematic flow chart of a method to determine overlay errorin accordance with a preferred embodiment of the present invention;

FIG. 15 is a schematic illustration of a preferred embodiment of thepresent invention as an inspecting tool;

FIG. 16 is a schematic illustration of one way by which light from alight source may be conveyed to the ring light;

FIG. 17 is a schematic flow chart of a method for both coarse and fineinspecting in accordance with a preferred embodiment of the invention;

FIG. 18 is a schematic illustration of the field of view of the opticalhead during OCD measurement;

FIG. 19 is a schematic illustration corresponding to that of FIG. 9, butshowing another form of optical system that may be used in the apparatusof the present invention; and

FIG. 20 is a schematic illustration, corresponding to FIG. 10, butshowing a modification that may be included when the optical system ofFIG. 19 is used.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 6 schematically illustrates an apparatus set-up corresponding tothe conventional one illustrated in FIG. 1 but modified to incorporatean integrated lithography monitoring (ILM) system in accordance with thepresent invention.

Thus, FIG. 6 illustrates apparatus for processing substrates, in thiscase wafers w, according to a predetermined photolithography process.This illustrated apparatus comprises: a loading station LS correspondingto the cassette loading station 6 a in FIG. 1, in which the cassettesare loaded onto the phototrack 5; a coating station CS, corresponding tocoater track 6 in FIG. 1 in which the wafers are coated with aphotoresist material; an exposure station ES, occupied by the exposuretool 8 in FIG. 1, in which the photoresist coating is exposed to lightthrough a mask having a predetermined pattern to produce a latent imageof the mask on the photoresist coating; a developing station DS,corresponding to developer track 10 in FIG. 1, in which the latent imageis developed; and an unloading station US, corresponding to station 10 ain FIG. 1, in which the cassettes are unloaded.

In accordance with the present invention, the apparatus illustrated inFIG. 6 is modified to include a monitoring station MS preferably betweenthe developing station DS and the unloading station US. The monitoringstation MS is occupied by an optical monitoring system, generallydesignated 14 in FIG. 6, which is an ILM (integrated lithographymonitoring) system, for measuring and/or inspecting the wafers w withrespect to predetermined parameters of the photolithography processafter the wafers have passed through the developing station DS andbefore reaching the unloading station US.

As will be described more particularly below, the ILM system 14 inspectsthe wafers w, immediately after processing, for one or more of thefollowing conditions after the wafers have passed through the developingstation DS and before reaching the unloading station US; (1) overlayregistration errors, in the alignment of the developed image produced onthe wafer in the respective photolithography process with respect to adeveloped image produced on the wafer in a preceding photolithographyprocess performed on the wafer, as described above in FIGS. 2 a and 2 b;(2) defects in the substrate, induced by the process malfunctions, aswell as dirt particles, etc., as described above; and/or (3) criticaldimension errors in the developed image of the photoresist coatingprocess produced during the respective photolithography process, as alsodescribed above. Preferably, the ILM system 14 performs all three of theabove functions, but in some applications, it may detect only one or twoor the above type errors.

The exact location of the ILM system 14 in the photocluster is governedby local considerations and circumstances, e.g., on the specificphotocluster tool manufacturer, the available foot-print inside thephototrack, and the Fab considerations.

FIG. 7 is a side view of the ILM system 14 according to a preferredembodiment of the present invention. The ILM system includes a rigid andstable supporting means 20 which receives and holds the wafer Wstationary. This can also be a vacuum supporting plate or a vacuumhandler (not shown) which holds the wafer W stationary from its bottom(back) side. Supporting plate 20 is preferably located between thedeveloping station DS and the unloading station US.

The ILM system 14 further includes a measuring unit (MU) 22 locatedabove supporting means 20. The measuring unit 22 and supporting means 20are rigidly mounted together in any suitable manner. As shown in FIG. 7,measuring unit 22 includes a sealed enclosure 21 with a transparentoptical window 37 aligned with and facing the supporting plate 20 andoptical unit inside the sealed enclosure 21, schematically indicated at23 having a movable optical head 24. The wafer is illuminated via asource 32 which is externally of the sealed enclosure 21 and directs abeam to the measuring unit 22 via an optical fiber (not shown) passinginto the sealed enclosure 21. As will be described more particularlybelow, movable optical head 24 enables the measuring unit 22 to performany one of a number of preselected measurements on any preselected waferW supported by plate 20 via transparent window 37.

As further shown in FIG. 7, the ILM system 14 further includes a controlunit, generally designated 26, which is externally of the sealedenclosure 21 and is connected to the MU 22 by electrical conductors (notshown) passing into the sealed enclosure 21. Control unit 26 includes acentral processing unit (CPU) 28, and optionally, an image processingunit (IPU) 30, as well as the electronic controls (not shown) forcontrolling the real time operation of the measuring unit 22.

Thus, in accordance with this preferred embodiment, the design of theILM system 14 should meet several principles, including: (a) small size,(b) maintaining the wafer stationary during measurement, (c) rigid andstable measuring unit, (d) cleanness restrictions attained by, amongother things, full separation of measuring unit 22 from the photoclusterenvironment i.e., all moving parts are located within the sealedenclosure 21 of the unit 22 and external light source, (e) high speedmeasuring (e.g., fast scanning), and (f) easy and quick maintenance by,e.g., simple replacement of any one of the above mentioned units. It isalso noted that the ILM system has the option to be bypassed by theproduction process and to be simultaneously operated in off-line or inintegrated modes.

FIG. 8 is a schematic illustration of the ILM system 14 according to apreferred embodiment of the present invention for measuring overlayregistration errors. However, it should be noted that the hereinafterdescription is applicable as well to other preferred embodiments of thepresent invention such as an apparatus for defect inspecting, or formeasuring OCD errors. The measuring unit 22 is shown in a measuringposition, i.e., above a wafer W with a developed PR coating 36. Theoptical head 24 is able to move rapidly along x and y axis of an X-Ystage 38, and also along the vertical Z-axis. Between the optical head24 and the wafer W there is the optical window 37 which prevents anypotential disturbance or contamination to the photocluster tools fromthe measuring unit 22.

The measuring unit 22 further includes a calibrating unit 40 whichsimulates a measuring position for the optical head 24 when it islocated above it. The calibrating unit 40 is composed of a target 42, aglass plate 44, and a mirror 46. The target 42 is any high contrastobject, such as a metallic pattern on a glass substrate which issuitable for determining the line spread function of the optical system(e.g., a knife-edge pattern). The glass plate 44 is of the same materialand thickness as optical window 37. The target is located in the objectplane of objective 76 similar to where the wafer W is located.

FIG. 9 is a schematic illustration of the measuring unit 22 according toa preferred embodiment of the present invention as an overlay metrologytool. However, the illustrated optical configuration is applicable aswell to other preferred embodiments of the present invention, such asfor defect inspecting, or for OCD metrology, in a manner to be discussedlater. As shown, the measuring unit 22 is composed of two alternativechannels inside the sealed enclosure 21: (a) an alignment orlow-magnification channel 62, and (b) a measuring or high-magnificationchannel 64. The low-magnification channel 62 is aimed at locating theoptical head at the right position above an overlay target (FIG. 2) tobe measured, whereas the high-resolution channel 64 is aimed at imagingthe overlay target. In this embodiment, a single external white sourcelight 32 and a single area CCD camera 92 serve both channels.

In another preferred embodiment of the present invention and for certainapplications, a filter(s) is added (not shown) after light source 32 inorder to produce a certain narrow spectral bandwidth which increases thecontrast of the features to be measured.

The low-magnification channel 62 comprises an objective 66, a beamsplitter 68, a shutter 70, a tube lens 72 and a beam splitter 74.Channel 62 has a relatively low-magnification power (e.g. 0.3-1.0×). Theobjective 66, which is part of the optical head 24 (FIG. 8), has a smallnumerical aperture and images a wide field of view (FOV) (e.g., 20-40mm).

The high-magnification channel 64 comprises a vertically movableobjective 76 which is part of the movable optical head 24 (FIG. 8), abeam splitter 77, a shutter 80, a tube lens 81, a beam splitter 90, afocus target 79, and LED illuminator 91. This channel has a relativelyhigh-magnification power (e.g. ×20-100). The objective 76 has largenumerical aperture since high resolution is needed and images arelatively small FOV (about 100 μm).

If higher accuracy is needed, measurement data correction may beachieved by determination of the actual incident angle of theilluminating light on the wafer's surface as illustrated in FIG. 10. Themeans for doing this is installed in the movable optical head 24 insidethe high magnification channel 64 and comprise an LED 93, two identicalmirrors 94 a and 94 b, two identical lenses 96 a and 96 b, and aposition sensor (electronic) device 98 composed of single suitablephotodiode or array of such photodiodes. The light from the LED 93 isreflected from the mirror 94 a and is focused by lens 96 a on the waferat the same location where the light from the objective 76 is focused.From there, it is brought back through lens 96 b and mirror 94 b to theposition sensor device 98. The position where the ray impulses theposition sensor device 98 is translated by means of a function to theangle β between the objective's chief ray 99 and the ray 97. Themeasured angle β is introduced during a later step of image processingin order to correct the inaccuracies which may arise during measurement.

The focusing target 79 (FIG. 9.) is any high contrast object, such as ametallic pattern on a glass substrate. The pattern can be any easilyidentifiable pattern, such as a contrast edge, a grid, etc. It isinstalled in the optical path with the option of removing it when notneeded (movable target), or of locating its image in the CCD plane 92 insuch a way which does not interfere with the imaged wafer or imagedoverlay targets. Other methods of focusing sensors can be applied aswell.

A selection is needed to enable selection between operating thealignment channel 62 or the measuring channel 64. In this embodiment theselection is realized by shutters 70 and 80 which can be selectivelyopened or closed.

Reference now is made to FIG. 11 which is a side view along the sectionline A-A in FIG. 2B. FIG. 11 illustrates the uppermost features layer100 on the wafer, and above it, the developed top PR layer 102. Layers100 and 102 are separated by the interface 101, whereas layer 100 isseparated from the layer below it (not shown) by interface 103. Layer100 comprises the overlay target lines 11 a and 11 b, whereas PR layer102 comprises the overlay target lines 16 a and 16 b.

The focusing procedure is aimed at locating, in a repetitive way, theobject plane 104 of objective 76 of the measuring channel 64 atpredetermined distances, Δz₁ and Δz₂, from interfaces 101 and 103,respectively. These distances are determined during the measurementprogram preparation for a certain product to be measured.

The focus condition of objective 76 over interface 101, indicated as z₁,in FIG. 11, is determined according to any known procedure, such as thatdisclosed in U.S. Pat. No. 5,604,344. In the same manner the objective76 is additionally moved down in order to detect, in the same procedure,its exact location z₂ above interface 103. It should be noted that whenthe overlay target 17 on layer 100 is a ‘negative’ feature (e.g., atrench), instead of ‘positive’ feature such as 11 a, it is possible todetermine the focus plane 104 with respect to plane 17 a instead ofinterface 103.

Once the locations z₁ and z₂ are known, the object plane 104 ofobjective 76 can be precisely located at distances Δz₁ and Δz₂ frominterfaces 101 and 103 respectively. At this location, measuring takesplace in order to produce an approximately equivalently defocused imageof both target lines 11 and 16 onto the CCD's image plane 92.

To calculate the overlay error, the exact locations of the centers oftarget lines 11 a, 11 b, 16 a, and 16 b should be determined. For thispurpose, several alternative methods are known. It is noted that withrespect to other types of overlay targets (e.g., multi-layer box, notshown) the same below-described methods can be used. One method isillustrated with the aid of FIG. 12 which shows the gray level of targetlines 11 a and 16 a (FIG. 11) images. The gray levels are obtained bytransforming the electrical signals of the CCD camera 92 (FIG. 9) intothe digital form, e.g., by means of analogue-to-digital converter (notshown). The Central Processing Unit (CPU) 28 (FIG. 7) determines thecenters of the gray levels lines 11 a and 16 a. The difference betweenthese centers Δx expresses the length of line 14 a (FIG. 2), dependingon the magnification along the measuring and imaging channels. In asimilar manner the length of line 14 b is determined and the overlayerror can be calculated for the x axis.

In the same manner, the overlay error can be calculated for the y-axis.

When the shapes of the gray levels 11 a and 16 a (FIG. 12) are notsymmetrical with respect to the vertical axis, or are imperfect, theoverlay error may be calculated using the line spread function (LSF) ofthe measuring channel. The LSF is accurately determined with the aid ofthe calibrating unit 40 (FIG. 8) at different heights above thecalibrating target 42. FIG. 13 illustrates at “A” the gray levels ofimages of a ‘knife edge’ pattern on a calibrating target at twodifferent heights 21 and 22 above the calibrating target 42. Thederivatives of the gray levels 21 and 22 with respect to the x-axis areshown at “B”, as 23 and 24, respectively, in FIG. 13. Now, by applying ade-convolution process the CPU 28 calculates the shape of the targetline along the x axis. In order to compensate for the physical height ofthe target lines, de-convolution is conducted in at various locationsalong the target line profile (vertical axis) using the suitable LSF, asshown at “B” in FIG. 13 for each location. The target shape isdetermined from these profiles.

In the same manner, the shape of the target lines along the y axis canbe determined, and the overlay error may be calculated.

In general, when the gray levels shapes 11 a and 16 a (FIG. 12) are notsymmetrical with respect to the vertical axis, or are imperfect, a morecomplicated algorithm can be used, e.g., a comparison of the obtainedgray levels of the target lines with their original shape and dimensionsin the masks.

FIG. 14 is a schematic flow chart of a method to determine overlay errorin accordance with a preferred embodiment of the present invention.After a new wafer to be measured arrives at the supporting plate 20(FIG. 7), calibrating of the measuring system takes place and then thewafer is aligned with respect to its principle axis. After alignment,the optical head 24 moves to a pre-determined site on the waferaccording to a previously prepared program. The program contains datawhich is relevant for operating the alignment 62 and measuring 64channels (FIG. 9), such as recognized patterns of the overlay targetonto the wafer and its coordinates. With the aid of the wide FOV of thealignment channel 62 (FIG. 9), and relevant data in the program, theoptical head 24 is brought above the site area. Now, a final alignmentcommences in order to locate the optical head 24 in its exact positionabove the site. An example of a method for wafer alignment (practicallyachieving both objectives of pre- and fine alignments) based on itspattern features is disclosed in U.S. Pat. No. 5,682,242. Then, theshutter 70 (FIG. 9) enters the optical path and blocks the alignmentchannel. An autofocusing mechanism in the optical head 24 focuses theobjective 76 (FIG. 9) on the focus plane 104 (FIG. 11). The overlaytargets are imaged on the CCD 92, and the data which was obtained isprocessed by the image processing unit 30 (FIG. 7) in order to determinethe overlay error. If all the predetermined sites on the wafer arealready measured, the wafer is released back to the phototrack 5, and anew wafer is brought to the supporting plate 20. If not, measurement ofthe next site on the wafer takes place.

It is noted that the overlay tool has various operational modes: (i)overlay error measurement; (ii) the same as in (i), and anothermeasurement when the wafer is rotated 180°; (iii) the same as in (i)conducted on one wafer, and another measurement conducted on anotherwafer which is rotated 180° with respect to the first; (iv) overlayerror is measured at different heights, and accuracy is determined byrotating the wafer.

According to another preferred embodiment of the present invention, theoverlay error data which is determined by the processing unit, istransferred to a general control unit 200 (FIG. 14) of the photocluster,or of a specific tool in the photocluster. General control unit 200 usesthis data for a feedback closed loop control to the exposure tool 8. Itcan also instruct the overlay metrology itself with respect to itsoperation (e.g., sampling frequency, site number to be measured on awafer).

It will be appreciated that, by combining the processed data from theoverlay system, with data of the defect inspecting process and of theOCD metrology, all within the same apparatus, an extensive integratedmonitoring and control system for the photolithography process can beestablished. It will also be appreciated that overlay errors, defectsand OCD errors can be determined during the production process itself,or after or before any predetermined step; and that all this can be doneeither on all wafers of a lot, or on several selected wafers in the samelot.

FIG. 15 illustrates a preferred embodiment of the present inventionconfigured as a defect inspecting tool.

The defect inspecting configuration is composed of: (a) two alternativeoptical channels, namely (1) a coarse inspecting channel 62 with a0.3-3.0× magnification, and a (2) fine inspecting channel 64 with >20×magnification; (b) a fast image acquisition system 320; and (c) aprocessing unit 26. According to this embodiment, the inspecting tool isrealized in the same overlay metrology, as described above.

With reference to the previously-described FIG. 9 which illustrates themain optical elements in the measuring unit 22, only a few opticalelements need to be added in addition to those used in the overlaymetrology system as described above to enable the apparatus also to beused for inspecting of defects. Thus, the same apparatus can serve bothas an overlay metrology tool or as a defect inspecting tool. Obviously,the defect inspecting functions can be realized in a separate apparatusthan the overlay metrology function.

The additional elements added to the measuring unit 22 of FIG. 9, forthe defect inspecting function, include a light source 300, shutters 302and 304 to block either the additional light source 300 or light source32, and a ring light 306 which surrounds the objective 66. Ring light306 has to produce a uniform light cone around the objective 66 with anopening angle of ca. 5-10°.

FIG. 16 illustrates how light from an external light source 300 isconveyed to the ring light 306. This is achieved by means of a bundle308 of optic fibers passing through the sealed enclosure 21, whereineach single fiber leads its light to a certain location onto theringlight. In the illustrated embodiment, ring light 306 is a fiberoptic ring light. As an alternative, LEDs with narrow bandwidths couldbe used when placed along the ring light periphery. The ring light isaimed at producing a uniform light-cone with an opening angle largerthan ca. 2° (α in FIG. 16) in order to cause diffracted non-specularlight from the wafer W to fill the objective 66.

Alternatively, light 310 coming through objective 66 from light source32 illuminates the wafer W and its specular component fills objective66.

Thus, in this preferred embodiment, illumination and viewing methods forcoarse inspecting 62 are alternatively bright (BF) and dark (DF) fieldsilluminations, using shutter means 302 and 304 to block either lightsource, or by turning on/off the electrical supply to the light source.The fine inspecting channel 64 is realized by BF illumination only.

It will be appreciated that illuminating and viewing, in general, can berealized by either BF or DF illuminations, all dependent on specificinspecting goals (e.g., defect type). Also during BF and/or DFillumination, for certain applications increased contrast can berealized for example, by additional filter(s) (not shown) after lightsources 32 and 300, respectively, in order to produce a certain narrowbandwidth. Further, during DF illumination, and for certainapplications, a better distinction between diffraction and scatteringeffects can be achieved, e.g., by alternating broad and narrow spectralband illumination.

The defect inspecting tool may be designed to meet the same principlesdescribed above with respect to an overlay metrology system.

FIG. 17 is a schematic flow chart of a method for both coarse and fineinspecting in accordance with a preferred embodiment of the presentinvention.

After a new wafer to be measured arrives at the supporting plate, thewafer is pre-aligned with respect to its principle axis, in order toparallel the wafer's scribe lines and the CCD's lines. An example for amethod for wafer's alignment disclosed in U.S. patent application Ser.No. 09/097,298. After pre-alignment, final alignment should take place,and a known method for this purpose based on its pattern features isdisclosed in U.S. Pat. No. 5,682,242. With respect to fine inspecting,final alignment is aimed at fine correlation of the predetermined siteto be inspected with its pattern stored already in the data base. Suchdata base is prepared, among other things, during recipe preparation.

After final alignment is conducted, image grabbing is performed duringcoarse inspecting in a step and repeat mode. According to this method,the optical head 24 moves to a predetermined area on the wafer, thenstops and stabilizes and an image is grabbed. The procedure is repeatedby moving to the next predetermined site usually used for waferinspection.

According to the present invention, step and repeat procedure allows fora better performance than using a linear scanning method, e.g., rasterscanning. During raster scanning, the wafer is continuously scanned andimages are simultaneously grabbed. This method suffers from severaldrawbacks, such as reduced resolution and blurring along the movementaxis, reduced resolution and inaccuracies due to non-stable velocity ofthe scanner, and non-efficient exploitation of the illumination system.During fine inspecting, images are grabbed at predetermined sitesaccording to the recipe. At the next step, each image is processed inorder to search for defects. This is performed either with absolute orcomparative methods as known in the prior art. The processed data isstored in a data base. When the whole wafer complete a coarseinspecting, or all the predetermined sites are finely-inspected, postprocessing commences. Alternatively, post-processing may be conductedsimultaneously. During post processing, the data can be evaluated andreported at different levels. This can be (a) defects list includingnumbers and coordinates of defects detected on the wafer, or (b) defectslist including coordinates and defects dimensions, or (c) defects listincluding coordinates and defects identification, or (d) morphologicaldefects analysis, e.g., according to local and/or overall waferdistribution, such as radial distribution which may point on poorspinning during coating. This can be followed by (e) photographingcertain defects for an additional processing; (f) attributingautomatically defects to a certain problem source; (g) and reviewingoptions for correcting the defects (all or part). In addition, coarseand fine inspection can be combined. According to the processed resultsof the coarse inspection, fine inspection may be conducted in certainsites on a wafer where it is likely to find (e.g., based on thresholds)certain defects.

The post processing data, which is determined by the processing unit,may be transferred to a general control unit 200 of the photoclustertool. General control unit 200 may use this data for a feedback, orclosed loop control, based on the level the data is processed (e.g.,defect identification, or cause analysis). The feedback may be sent tothe coating or other station which may affect the phototrack 5. Thefeedback may also instruct the inspecting metrology system itself withrespect to its operation (e.g., sampling frequency, sites number to bemeasured on a wafer).

It is evident that these embodiments are superior to a parallel‘stand-alone’ system in general, and to a visual inspecting system.

For certain occasions, where a more detailed inspecting is needed theILM system 14 can be used as an off-line system so as not to disturb theproduction process.

According to another preferred embodiment of the present invention, theabove-described overlay metrology system can also be used as OCDmetrology system. The OCD metrology system as illustrated in FIG. 9would contain the same channels 62 and 64 as the overlay metrologysystem, as well as the other optic elements. OCD determination would beexecuted in a similar way as overlay error determination as shown inFIG. 14 and discussed above. For this purpose, the optical head 24 (FIG.9) would be moved to a predetermined site by the alignment channel 62,and then the measurement channel 64 would be operated in order to imagethe features to be measured.

FIG. 18 illustrates the FOV 301 of the optical head 24 duringmeasurement. In this example, the FOV contains two typical features tobe measured: line width 312 and space 314. It is noted that according tothis method for OCD determination, the FOV 301 should include a set ofidentical features to be measured. If this set is not part of theoriginal features on the wafer, a test site which includes set(s) ofsuch features should first be prepared.

Since the features to be measured are located in the same layer,focusing is conducted similarly to overlay measurements as describedabove, however, only with respect to one layer, except when features tobe measured are in different layers. The feature's shape isreconstructed, in the same manner as for overlay, from its image usingthe LSF (x or y, z) of the optical system. The width of a space isdetermined by its adjacent lines edges which are reconstructed.

In this method, the width of the identical features in the set aredetermined, and by applying statistical calculations (e.g., mean value)the width of a representative feature is calculated. The accuracy ofthis measurement is based on the feature's degree of symmetry, theoptical system, and the number of identical features to be measured. Itis to be noted that usually, only the two latter parameters can beadjusted and prepared in advance, according to specific circumstances,in order achieve the desired accuracy.

The monitoring and control based on OCD is established in the samemanner described above with respect to overlay error determination.

FIG. 19 illustrates a modification in the optical system of FIG. 9; andFIG. 20 illustrates a modification in the system of FIG. 10 when usingthe optical system of FIG. 19. In this preferred embodiment, theinternal configuration of FIG. 8 is slightly different. The MU 22includes a sealed enclosure 421 with an optical window 437, where insideare installed the optical head 424 (modified according to FIGS. 19 and20), the optical head's positioning means along x,y, axis 38 as well asalong z axis, the calibrating unit 40, and additional electronicfeatures and optic guides (not shown) which enable, respectively,external electric and light supply, as well as communications means (notshown) to the MU 22 with the CPU 28.

In the modified system of FIG. 19, the low-magnification channel 462,and the high-resolution channel 464, as well as the focusing target 479and the LED 491 (corresponding to channels 62, 64, target 79 and LED 91in FIG. 9) are contained within the movable optical head 424. It shouldbe noted that the low-magnification channel 462 is used either forpositioning the optical head 424 above a pre-selected site on a wafer tobe measured during overlay, fine inspecting and OCD applications, or forcoarse inspecting. The high-magnification channel 464 is used formeasuring during overlay, OCD and fine inspecting applications.

Light from the external light source 432 is conveyed by optical fiber438 and is split into two branches 438 a, 438 b, conveying the lightinto the sealed enclosures 421. Each of branches 438 a and 438 b isselectively controlled by shutters 470 and 471. Inside the sealedenclosure 421 the light from the two branches is conveyed by mirrors tothe diffusers 450, 451 of the low-magnification channel 462 and thehigh-magnification channel 464, respectively.

The low-magnification channel 462 includes a field lens 466, a beamsplitter 468, an imaging lens 472, a folding mirror 474, a beam splitter490, and the CCD 492. The alignment channel 462 has a relatively lowmagnification power, (e.g., ×0.1-1.0.); and the imagining lens 472 has asmall numerical aperture and images a wide field of view (e.g. 20-40mm).

The high-magnification channel 464 comprises an objective 476, a beamsplitter 477, a tube lens 481, a beam splitter 490, and the same CCD492. This channel has a relatively high magnification power (e.g.×20-100); and the objective 476 has a large numerical aperture sincehigh resolution is needed.

In this preferred embodiment, DF illumination is realized by a ringlight430 and light sources, e.g., LEDs, placed along the circumference of thering light 430 and electric wires to operate the ring light 430. Thering light 430 is aimed at producing uniform light-cone with an openingangle larger than ca. 2° in order to cause diffracted non-specular lightfrom the wafer w to fill the imaging lens 472. Light source 430 can beswitched by turning on/off the electricity supply.

If higher accuracy during measurement is needed, a system similar tothat illustrated in FIG. 10 may be used for accurately determines theactual angle between the optical axis 464 and the wafer surface W. FIG.20 illustrates such a system which, in this case, is installed inside ahousing 425 which surrounds the objective 476, and includes an LED 493,two identical mirrors 494 a, 494 b, two identical lenses 496 a, 496 b,and an electronic position sensor 498, corresponding to elements 76, 93,94 a, 94 b, 96 a, 96 b, 98, respectively, illustrated in FIG. 10. Thesystem in FIG. 20 can measure the angle β between the normal ray and theray 497 from which the angle between the optical axis 499 and thewafer's plane can be determined.

The system of FIGS. 19 and 20 are otherwise constructed and operated insubstantially the same manner as described above with respect to FIGS. 9and 10, and utilize the focusing target 479 and calibration unit 40 formeasuring channel 464.

Selection of the positioning mode of operation utilizinglow-magnification channel 462, or the measuring mode of operationutilizing high-magnification channel 464, is realized by operating themechanical shutters 470 and 471. The focused condition for the measuringchannel 464 is determined according to known procedures, such as thosedescribed in the above-cited U.S. Pat. No. 5,604,344.

The modified optical system illustrated in FIG. 19 may also be used fordetermining overlay error in accordance with the flow chartschematically illustrated in FIG. 14. After a new wafer to be measuredarrives at the supporting plate 20, calibration of the measuring systemtakes place by identifying a predetermined site on the wafer W, andlocating the optical head 424 above it. The identification of apredetermined site may be based on the wafer pattern features, asdisclosed in the above-cited U.S. Pat. No. 5,682,242.

The modified optical system illustrated in FIG. 19 may also be used forOCD measurements and inspecting in accordance with the flow chartschematically illustrated in FIG. 17.

While the invention has been described with respect to several preferredembodiments, it will be appreciated that these are set forth merely forpurposes of example, and that many other variations, modifications andapplications of the invention may be made.

1. An apparatus for photolithography processing substrates, comprising:a loading station in which the substrates are loaded from at least onefirst cassette; a coating station in which the substrates are coatedwith a photoresist material; a developing station in which the latentimage is developed; an unloading station in which the substrates areunloaded; and a monitoring station for monitoring the substrates withrespect to predetermined parameters of said photolithography process andbefore being unloaded in at least one second cassette at said unloadingstation, said monitoring station comprising: a supporting plate forhandling substrates to be monitored; an optical monitoring system formonitoring substrates on said supporting plate, the optical monitoringsystem comprising a low-magnification channel for aligning said opticalmonitoring system with respect to a substrate on said supporting plate;and a high-magnification channel for measuring said predeterminedparameters of the photolithography process after the substrate haspassed through said developing station and before reaching the unloadingstation; and an alignment system for providing monitoring apre-determined locations of substrates.
 2. The apparatus according toclaim 1, wherein: said apparatus further includes a transfer device fortransferring said substrates from one station to another; and saidtransfer devices also transfers said substrate to said monitoringstation.
 3. The apparatus according to claim 1, wherein said monitoringstation is between said developing station and said unloading stationfor monitoring the substrates after the substrates have passed throughthe developing station and before reaching the unloading station.
 4. Theapparatus according to claim 1, wherein said monitoring station includesan optical monitoring system for detecting overlay registration errorsin the alignment of the developed image produced on the substrate in therespective photolithography process with respect to a developed imageproduced on the substrate in a preceding photolithography processperformed on the substrate.
 5. The apparatus according to claim 1,wherein said optical monitoring system is configured for detectingdefects in the substrate, including defects produced by thephotolithography process performed on the substrate.
 6. The apparatusaccording to claim 1, wherein said optical monitoring system isconfigured for detecting critical dimensional errors in the developedimage of the photoresist coating produced during the respectivephotolithography process.
 7. The apparatus according to claim 1, whereinsaid optical monitoring station comprises an optical system definingsaid low-magnification and high-magnification channels located in anenclosure with an optical window; and a light source is external to saidenclosure and produces a light beam which is applied to said opticalsystem within said enclosure.
 8. The apparatus according to claim 1,wherein: said optical monitoring system includes an optical imagingdevice; and said monitoring station further includes an optical imageprocessing unit connected to said optical imaging device by electricalconductors.
 9. The apparatus according to claim 1, wherein saidmonitoring station further includes a central control and processingunit connected to said optical monitoring system for controlling saidsystem via electrical conductors.
 10. The apparatus according to claim1, wherein both said channels are fixed with respect to each other andwith respect to said transparent window.
 11. The apparatus according toclaim 1, wherein said optical monitoring system further includes amovable optical head and containing an objective lens for saidlow-magnification channel, and an objective lens for saidhigh-magnification channel.
 12. The apparatus according to claim 11,wherein, said lenses are movable together upon moving said optical head.13. The apparatus according to claim 12, wherein the objective lens ofsaid low-magnification channel has a relatively small numericalaperture, and the objective lens of said high-magnification channel hasa relatively large numerical aperture.
 14. The apparatus according toclaim 13, wherein said high-magnification channel further includes ameasuring device within the optical head for accurately measuring theangle of incidence of the light source with respect to the surface ofthe substrate in the monitoring station.
 15. The apparatus according toclaim 14, wherein said measuring device comprises: an identical mirrorand an identical lens on each side of the objective lens of thehigh-magnification channel; an LED illuminator on one side of the latterobjective lens for projecting light through the lens and mirror on therespective side of objective lens onto the substrate, and then throughthe lens and mirror on the opposite side of the objective lens; and aposition sensor on said opposite side of said latter objective lens. 16.The apparatus according to claim 1, wherein each of said channelsincludes a shutter between a light source and the channel, said shuttersbeing selectively opened and closed to selectively enable one of saidchannels.
 17. The apparatus according to claim 1, wherein said opticalmonitoring system a calibrating unit which simulates a measuringposition for the optical head when the optical head is located inalignment with the calibration unit.
 18. The apparatus according toclaim 17, wherein said calibration unit includes a target of a highcontrast material, a glass plate, and a mirror.
 19. The apparatusaccording to claim 1, further comprising an independent control unit.